Mr Marek set off, with his brother in tow, armed with vague geographical information from 1920s research and topographical maps to see if they could pinpoint the mysterious animal.

"My brother and I were looking around and we both spotted it at the same moment. It was just staggering," he said.

Over the course of three field trips, researchers unearthed 12 of these cream-coloured, leggy arthropods - a class of creatures with segmented bodies and jointed legs that includes spiders, insects and crustaceans.

The captured females were found to have between 662-666 legs, over 170 segments and measured about 33mm (1.3in) long. The males, in comparison, were about half of the females' length with between 318 and 402 legs.

All of the specimens measured a thread-like 0.57mm (0.02in) in width.

The scientists believe the varying lengths are explained by the fact that millipedes continue to grow additional segments and legs even after they have reached sexual maturity - so the 750-legged millipede found in 1926 may have been an older specimen.

Fragile habitat

The millipedes also showed some other unusual features when examined more closely.

Each segment has an intricate surface texture

"We looked at them under a scanning electron microscope and we found a very complex and intricate morphology, including spikes and spines, and also the presence of setae (a type of bristle) on the back of the millipedes that secrete this silk-like substance," Mr Marek said.

"These were found on every segment on their backs, and we have no idea what this silk does."

I. plenipes is only thought to exist in a 0.8sq km (0.3sq mile) area of the biodiversity hotspot California Floristic Province. The team has called for urgent protection of the rare creature's fragile habitat.

But the researchers say the species' location may also point to clues about its past.

Other closely related species belonging to the same family as I. plenipes, Siphonorhinidae, are found in other narrow geographical ranges in southeastern Africa, Sumatra, Indo-Burma, Sumatra, Malaysia and Java.

The team believes this unusual distribution hints that the age of this particular family pre-dates the break-up of super-continent Pangaea over 200 million years ago.